Final answer:
Marcus is likely experiencing Panic Disorder, as his symptoms align with the sudden and intense characteristics of panic attacks, unlike other anxiety disorders with different features. Therefore, the correct option is c.
Step-by-step explanation:
Marcus appears to be suffering from Panic Disorder, characterized by unpredictable episodes of intense fear or discomfort, where symptoms such as difficulty breathing and feeling like he's having a heart attack emerge abruptly and reach a peak within minutes. This form of anxiety disorder differentiates from Generalized Anxiety Disorder, Phobia, and Post-Traumatic Stress Disorder (PTSD) which have distinct features and triggers. Panic Disorder focuses specifically on the suddenness and severity of the anxiety episodes without an identifiable trigger.